The basic events associated with the generation of a mutagen-induced chromosome aberration are not understood. The premise which underlies this proposal is that an important approach to obtaining that understanding is to characterize the sites at which chromosomes become broken. One goal of the project is to determine if there are "hot spots", sites at which chromosomes have a high tendency to break. This will be achieved by studying the distribution of approximately 200 chromosome breaks over a region of about 250 Kb of Drosophila melanogaster DNA. Breakpoints will be positioned on a restriction map of the entire region encompassing cytogenetic positions 14B3-14B18 and 14F1-15B2. A second goal of the project is to compare the distribution of radiation-induced breaks to those which are chemically-induced. Finally, this project will address the question of what precisely happens to DNA sequence at a chromosome break. Do the two broken ends which become attached undergo any processing that alters DNA sequence i.e. duplication, small deletions, or other alterations. This question will be answered by sequencing through several break points and comparing these sequences to the wild type DNA of the position where the breaks occurred.